1. Technical Field
This disclosure generally relates to a light adjustment circuit for alternating-current light emitting diodes (AC-LED's) and, more particularly, to a light adjustment circuit capable of adjusting the brightness of AC-LED's in correspondence to variation in input AC power to enable each of the AC-LED's to achieve a predetermined brightness with enhanced reliability and lifetime.
2. Technical Background
Unlike conventional direct-current light emitting diodes (DC-LED's) that require an AC-to-DC power supply, alternating-current light emitting diodes (AC-LED's) are novel and promising because they are directly driven by an AC power supply with higher efficiency and reliability than conventional DC-LED's.
However, variations in voltage and frequency of the input AC power supply affect the opto-electronics characteristics of the AC-LED's to cause unstable brightness of the AC-LED's since the AC-LED's are directly coupled to the AC power supply.
In view of the above, it is generally that a current-limiting resistor is serially connected so as to limit the current from the AC power supply and protect the AC-LED's from being burnt out. However, such a current-limiting resistor is only capable of limiting over-current and is unable to overcome the problems, such as affected output characteristics, due to variations in voltage and frequency of the AC power supply. Moreover, the current-limiting resistor causes power loss that lowers the power conversion rate and is unable to achieve light adjustment circuit for the AC-LED's.
Therefore, U.S. Pat. Pub. No. 2004/0206970 discloses an LED driving circuit as shown in FIG. 1. The LED driving circuit comprises a full-bridge rectifier 12 and an output filter capacitor 13 so as to rectify the AC power from an AC power supply 11 and provide a load (such as an LED) 14 with stable DC power. The output filter capacitor 13 filters the rectified voltage to provide stable DC power supply to the LED 14 even though the input AC power may be unstable. However, since the driving circuit provides DC power to the load, the DC power may be insufficient for AC-LED's as the load to cause lowered brightness. Meanwhile, the driving circuit is unable to achieve light adjustment circuit for the AC-LED's and to turn off the AC-LED's when the input voltage is too high.
U.S. Pat. No. 7,489,086 discloses an LED driving circuit as shown in FIG. 2. The LED driving circuit comprises a fixed high-frequency inverter 22 capable of converting the power from the input AC voltage or DC voltage 21 and providing a relatively constant voltage and relatively constant frequency output 24 to be delivered to a load (such an AC-LED). The ED driving circuit further comprises an AC regulator 23 that senses changes to the load at the output lines of the inverter 22 and feeds back a voltage signal to the inverter 22 in response to changes in the load which makes adjustments accordingly to maintain a relatively constant voltage output with the relatively constant frequency output. However, the LED driving circuit is complicated with high cost and is unable to turn off the AC-LED when the input voltage is too high.
TW Pat. Appl. No. 095146805 discloses an LED driving circuit as shown in FIG. 3. In FIG. 3, an AC-LED array 31 disposed between two terminals 33 and 34 is connected in series or in parallel with an element 32 being a resistor, a capacitor, a Zener diode or a surge absorber so that the LED driving circuit provides temperature compensation, voltage correction or surge protection.
In addition, phase control has been reported using a zero voltage detection circuit to achieve light adjustment for AC-LED's. However, it fails to turn off the AC-LED's when the input voltage is too high.
This disclosure provides a light adjustment circuit for AC-LED's, capable of adjusting the brightness of AC-LED's in correspondence to variation in input AC power to enable each of the AC-LED's to achieve a predetermined brightness with enhanced reliability and lifetime.